DIGESTION 121 



exception to the rule that digestibility is diminished by cooking. 

 Similar data may be obtained for all foods. They are no doubt 

 useful indications of the course of action which we may expect 

 to occur within the stomach, but we can never be sure that 

 my lord will obey the ruling of the chemist. Practice with a 

 captive golf-ball is a useful preparation for the game ; but there 

 are conditions on the links which cannot be reproduced on the 

 lawn. In an artificial stomach the clean fibre of raw fish 

 digests more slowly than raw beef. Even when the beef is 

 roasted and the fish fried or boiled in the ordinary way, the 

 beef disappears through the dialyser (the bag of membrane 

 suspended in a vessel of warm water in which experimental 

 digestion is carried out) more quickly than the fish. Never- 

 theless, the living stomach is better disposed towards a mixed 

 meal containing a certain weight of fish than towards a meal 

 in which, the other constituents remaining the same, beef takes 

 the place of fish. Important conclusions may, no doubt, be 

 drawn from observations of the time occupied in the peptoniza- 

 tion of pure food i.e., fibrin, white of egg, clean meat, etc. 

 under conditions simulating those which are present in the 

 stomach ; but they must be accepted with many reservations. 

 In the stomach it is not pure substances, but mixtures, that the 

 gastric juice has to deal with. And here a most important 

 factor comes into play, to which further reference will be made 

 later on. The amount and quality of the secretion of the 

 gastric glands depends upon the nature of the food. Hence a 

 food, or a combination of foods, which digest readily in 

 the laboratory may take a long time to disappear from the 

 stomach, and vice versa. Digestibility depends upon the 

 nature of the food. It depends also upon its physical state. 

 To take simple illustrations : Cheese contains coagulated casein, 

 one of the most easily digestible of proteins, but the casein is 

 intimately mixed with fat, upon which gastric juice can make no 

 impression. Even when finely divided, the particles of casein 

 are protected from the action of the juice by fat. In the same 

 way the meat of pork is as digestible as mutton, but the fat 

 of pork is quickly melted and very liquid. In the process of 

 cooking the muscle-fibres become saturated with fat. 



It is not the function of the stomach to complete digestion. 

 Its business is to initiate it. Food which reaches the stomach 



